The present invention relates to wire for fabricating lamp filaments, particularly to such wire fabricated from a tungsten alloy, and to processes for producing the alloy and the wire.
A filament for an incandescent lamp with high vibration resistance must have a microstructure specifically tailored to resist fracture caused by vibration of the lamp. Such vibration resistant microstructures typically include a high proportion of elongated grains oriented in the longitudinal (axial) direction, with several elongated grains across the diameter of the filament wire and long segments of grain boundaries running parallel to the filament wire axis. This type of microstructure is distinct from an equiaxed microstructure, which exhibits only short segments of grain boundaries running parallel to the wire axis. The abundant long grain boundaries in the highly vibration resistant microstructure act effectively as vibration dampeners, reducing the tendency of the filament wire to fracture.
The microstructure of a filament for a highly vibration resistant lamp is also different from that of a standard incandescent lamp. The standard incandescent lamp performs best when the filament during operation has a good non-sag microstructure. A typical non-sag microstructure is characterized by being largely free of grain boundaries, with an occasional wire segment including a long grain boundary running parallel to the wire axis. This type of non-sag microstructure is called an interlocking grain structure.
Prior to the present invention, three types of wire have been used for vibration resistant lamp filaments: a type of non-sag wire having a degraded non-sag microstructure, a tungsten-based wire including 3 weight percent rhenium, and a tungsten-thorium oxide wire. The degraded non-sag wire is the most readily fabricated and least expensive of the alternatives. However, it is used only for the least severe applications, since it does not perform as well as the other alternatives. The tungsten-rhenium wire is used for applications where the filament temperature is the highest, and for alternating current applications where the wire diameter is finer than for typical direct current applications. Tungsten-thoria wire is used for most other applications because it performs well and is less expensive than the tungsten-rhenium wire. However, the thorium in the tungsten-thoria wire is a radioactive material. Because of the radioactivity of thoria, the cost of manufacturing the alloy is increased. Care must be taken at each step to limit exposure of the workers to radioactive dust. Additionally, scrap generated in the process must be disposed of as low level radioactive waste in an appropriate disposal site. Thus the disposal cost is much higher than that for non-radioactive tungsten scrap, which can be recycled.
It would be desirable to have a readily fabricated, relatively inexpensive lamp filament of non-radioactive materials exhibiting excellent vibration resistance at high operating temperatures. The filament wire described herein was developed to address that need.